PM reduction is as important as NOX reduction in exhaust gas regulations of diesel engines, DPF is known as an effective technique in this regard.
DPF is a PM collecting device that uses a filter. As the PM continues to accumulate in the DPF in engine operating conditions with low exhaust gas temperatures, forced regeneration is carried out wherein the temperature is forcibly raised to burn the PM.
Common means of raising the temperature include delaying the fuel injection timing, post-injection, and intake throttling, which all encompass the problem of adversely affecting the fuel economy. On the other hand, higher temperatures mean a quick and efficient forced regeneration of DPF with a smaller decrease of fuel economy since the higher the temperature, the higher the speed of burning the PM.
However, if the DPF temperature is too high, the PM burn rapidly and the DPF temperature rises quickly, which may damage the DPF or deteriorate the catalyst carried in the DPF.
Temperature control is therefore necessary, to maintain the DPF temperature at a level suitable for the regeneration, so as to prevent a drop in the fuel economy and ensure safe regeneration of the DPF.
There is Japanese Patent Application Laid-open No. 2005-320962 (Patent Document 1) as an example of temperature raising control in forced regeneration of DPF. The Patent Document 1 describes a process of temperature control during regeneration of DPF wherein an optimal feedback gain in accordance with the operating condition is used to achieve both of stability and responsivity of temperature feedback control to raise the temperature to a target level.
There is a time delay between variable manipulation to raise the temperature and a change in the exhaust gas temperature. The time delay of control targets also varies depending on the changes in the operating condition. For example, an increase in the exhaust gas flow rate increases the heat transfer coefficient and decreases the time delay, while a decrease in the exhaust gas flow rate increases the time delay between changes in variable and changes in exhaust gas temperature as well as the time constant, whereby the time delay is increased.
Described in the document is a corrective action performed in consideration of the time delay to make the temperature closer to the target level quickly, wherein the operating condition is detected to determine the current time delay from the relationship between the exhaust gas flow rate and the time delay that is known from the operating condition, an optimal feedback gain is calculated in accordance therewith, and the temperature raising variables are corrected using this feedback gain.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-320962
Patent Document 1 describes making the temperature closer to a target level quickly by correcting a feedback gain as described above. However, it is not possible to improve the stability of control of the DPF inlet temperature by a corrective action using only a feedback gain, particularly when the exhaust gas flow rate has decreased, since, in such a condition, the time delay between changes in variable (post-injection amount) and changes in exhaust gas temperature is increased, and so is the time constant, because of which the exhaust gas temperature control performance is deteriorated, i.e., it takes long until a change appears in the DPF inlet temperature even when, for example, the post-injection amount is excessive.
If the temperature is controlled properly by feedforward control at various operating condition points, the feedback variables will be zero in a steady state and the problem associated with the feedback control described above will not arise. In a small general-purpose engine, however, in which the rpm and the load change independently in use, it is difficult to set the feedforward variables properly in all operating conditions.
If, for example, the flow rate of the exhaust gas has dropped largely in a short time, and the flow rate remains low after that, the DPF inlet temperature will rise, and when this phenomenon appears, it is difficult to solve it by improvement of the gain in the feedback control, or by optimization of controlled variables in the feedforward control.